Fix cursor position

An Atom release or two ago, I noticed that the click-to-navigate functionality in [go-plus](https://github.com/joefitzgerald/go-plus) had stopped working properly. After some tracing around I narrowed the problem down to this.

The main issue was that `activeFile` was now being populated with some kind of path with `..` components, rather than the expected project-relative path. I found [a better method in the Project API](https://atom.io/docs/api/v0.125.0/Project#instance-relativize) for this.

While I was changing that, I also used [the Editor's `setCursorBufferPosition` method](https://atom.io/docs/api/v0.125.0/Editor#instance-setCursorBufferPosition) to reposition the cursor. This way, it'll behave a little more nicely with multiple cursors active, by consolidating them into one at the new location.